This invention relates to improving coated electrode surfaces, and in particular to a method useful in surge limiters to minimize filament formation and ensure a good bond between the coating and electrode over essentially the entire interface.
Surge limiters are primarily used to protect apparatus from high voltage surges resulting from a variety of causes, such as lightning strikes. The devices basically comprise a pair of electrodes with a spark gap therebetween. The device is coupled in parallel with the protected apparatus and does not interfere with the functioning of the apparatus since the device is nonconducting during normal operation. However, when a voltage surge of sufficient magnitude appears at the electrodes, a spark is produced across the gap, and the surge is shunted from the apparatus. In a sealed gas surge limiter, the electrodes are placed in a hermetically sealed housing which includes a suitable gas. The device fires when the gas in the gap area is sufficiently ionized to produce a spark.
It has been recognized that a coating of graphite on the surface of the electrodes will improve device performance by increasing electron emission and thereby enhancing plasma discharge in the gap. However, a device with the as-deposited, unbonded carbon has a relatively short life. Also, in a narrow gap device carbon filaments tend to form on the surfaces of the electrodes after a few discharges of the device, and this effect results in leakage currents and could produce short circuits in some cases.
It has also been recognized that the bond between the coating and electrode could be improved by applying to the electrode a signal which causes conduction in the arc mode for several short periods of time. It was discovered that under the appropriate conditions, the spark would "dance" around the surface of the electrodes, causing a different portion of the coating to bond with the cathode during each conduction period. It was therefore suggested that a pulsed signal be applied to the electrodes with appropriate reversal of polarities until the entire surface of both electrodes was bonded (see U.S. patent application of P. Zuk, Case 14, filed on an even date herewith and assigned to the same assignee, which is incorporated by reference herein).
In a commercial environment, for long life, it is desirable to optimize this process by ensuring a uniform and complete reaction of both electrode surfaces within a reasonable time. At the same time, if the surface of the electrodes is too smooth, devices have a tendency to exhibit high surge limiting voltages. It is therefore also desirable to leave some asperities on the electrode surfaces to increase field emission and thereby ensure a low surge limiting voltage.